1. Field of the Invention
The present invention relates generally to a hydraulic valve lifter for an internal combustion engine for adjusting valve clearance of intake and exhaust valve.
2. Description of the Background Art
Japanese Utility Model First (unexamined) Publication (Jikkai) Showa No. 60-49206 discloses one of typical construction of valve lifter which forms background art of the present invention. The valve lifter comprises a hollow lifter body and a plunger assembly housed within the lifter body. The plunger assembly comprises an upper side opened and essentially cylindrical plunger body and a seat plunger disposed within the plunger body in mutually thrusting fashion. The plunger body and the seat plunger are cooperated with each other for defining a high pressure chamber therebetween. The seat plunger is formed with a hollow chamber serving as a reservoir chamber which communicates with the high pressure chamber via a check valve assembly for compensating working fluid pressure in the high pressure chamber.
The seat plunger has upper end mating with upper end wall of the lifter body, which upper end wall is formed with a through opening for draining part of the working fluid and ventilating air fed with the working fluid. In order to adjust working fluid flow rate, a metering ring plate is provided in opposition to the mating end of the through opening. The metering ring plate has an arcuated section opposing to the inner end of the through opening in spaced apart relationship for defining therebetween a clearance for permitting part of the working fluid and air to flow through the through opening.
In order to supply the working fluid to the reservoir chamber of the seat plunger, an annular fluid path is formed around the upper end of the seat plunger, which annular path is connected to a pressurized working fluid source. The annular path is in communication with the reservoir chamber via radial path including cut outs formed through the upper end portion of the seat plunger.
With the conventional valve lifter construction, the seat plunger contacts the inner surface of the upper end wall at the annular end surface which is discontinued by the cut out. To this contacting surface, substantially high pressure is repeatedly exerted while the engine is driven via a valve stem of the associated intake valve or exhaust valve. Therefore, the upper end edge of the seat plunger may cause wearing due to repeated exertion of substantial pressure. Since the clearance between the metering plate and the inner end of the through opening is maintained by the height of the upper edge of the seat plunger, wearing of the upper edge of the seat plunger may cause reduction of the clearance.
Reduction of the clearance may cause degradation of air ventilating performance to cause the air to flow into the high pressure chamber. Air in the high pressure chamber may cause instability of the valve action resulting in instability of the engine operation. In the worst case, the metering ring plate may completely close the inner end of the through opening to cause accumulation of the air in the reservoir chamber to prevent the working fluid from entering into the reservoir chamber. This results in lack of pressure in the high pressure chamber for causing incapability of valve clearance adjustment.